JWST ETC Coronagraphy Strategy

The JWST Exposure Time Calculator (ETC) Coronagraphy strategy is used for calculating the signal-to-noise ratio and contrast for MIRI and NIRCam coronagraphy ETC calculations.

Words in bold are GUI menus/
panels or data software packages; 
bold italics are buttons in GUI
tools or package parameters.

For coronagraphic ETC calculations, a user would be interested in the signal-to-noise ratio (SNR) that can be achieved for a faint companion that is close to a bright star, or for circumstellar features around a bright host star. The bright star in this case can be many orders of magnitude brighter than the faint features of science interest. The ETC provides predictions for coronagraphic observations by including reference PSF stars for PSF subtraction from the source of interest.

There are 2 tabs under Strategy for coronagraphy in the ETC: (1) Observation and (2) Extraction

Figure 1. Layout of the Observation under the Strategy tab for Coronagraphy

(1) Observation: Parameters for the PSF calibration are set under this tab.

  • Scene rotation: This allows rotation of the scene with respect to the non-azimuthally symmetric structure of the coronagraph's focal plane (such as cross pattern for the MIRI FQPM, or bars for NIRCam). When used in conjunction with the coronagraph visibility tool (at the page JWST Coronagraphic Visibility Tool Help), this feature allows the user to determine the optimal aperture position angle special requirement for the scene of interest.

  • PSF subtraction source: The reference star that should be used for PSF subtraction can be selected from the drop-down menu that has a list of all the sources in the scene.

    In order to have correct SNR and information in the report (e.g., saturation) on the science scene only and/or the PSF Subtraction Source only, the latter needs to be positioned away from the science scene (e.g., by 10 arcsec). This is done via the Offset sub-tab under Scene or using the Source Editor when defining the scene(s) and sources. Failure to offset the PSF Subtraction Source will result in SNR and other values corresponding to 2 overlapping sources.

  • PSF subtraction: The default PSF subtraction strategy, whereby 4 options are available:
    • Optimal (PSF Autoscaling): (Default) assumes that thermal and dynamical changes do not occur in the optical system between observations of the science target and the reference PSF star exposures. Residual noise, in this case, is driven solely by the shot noise in the wings of the host star and reference PSF source profiles, and SNR for the detection of the faint source depends on the exposure time. Ideally, the reference PSF star would be brighter and will be re-scaled for PSF subtraction. However, as currently implemented in the ETC, the user must set the magnitude and stellar type of the reference star to closely match that of the target star in order to yield a realistic result. Choosing different magnitudes and/or stellar types for the target and reference star will not trigger a warning in the ETC, but the user should be aware that such results might not be accurate.

    • Optimal (No PSF Autoscaling): will not scale the PSF subtraction source to match the central source before subtracting.

    • Unsubtracted Science Scene: displays only the science scene, with no subtraction and only the coronagraphic mask suppressing the central source. 

    • PSF Subtraction Source Only: displays the PSF subtraction source by itself, under the coronagraphic mask.
      Note: contrast and SNR estimates are reported for all options, though they are not particularly meaningful for the Unsubtracted Science Scene or PSF Subtraction Source Only options (particularly the PSF Subtraction Source Only option, where the SNR source is not present). Those two options are primarily provided for users who wish to download (separately) the simulated data and analyze the observations themselves. It is particularly useful, for instance, for users who wish to use a brighter reference PSF star with possibly its own set of readout parameters. 

Displays in the Images (2D SNR, Detector, Saturation, and Groups Before Saturation) and Reports (Report, Warnings, Downloads) panes change depending on which PSF subtraction option is selected.

Figure 2. Layout of the Extraction under the Strategy tab for Coronagraphy

(2) Extraction: The details of the extraction aperture and background subtraction are set under this tab.

  • SNR source: From the drop-down menu, which lists all the sources in the scene, the user can choose the faint source for which the SNR has to be calculated. The SNR source must lie within a square, centered on the coronagraphic mask and aligned with the detector rows and columns, with sides of 101 pixels for NIRCam or 81 pixels for MIRI.

  • Aperture radius: The radius of the circular extraction aperture used in this strategy.

  • Contrast azimuth: This sets the angle at which the contrast plot will be computed, measured counterclockwise from the positive Y axis, and is expected to be the angle along which the faint source of interest is located.

  • Contrast separation: This is the separation along the direction specified by the azimuth angle at which the scalar values are reported.

  • Angular units: This is fixed as arcseconds and determines the units for Aperture radius, Sky annulus, and Contrast separation.

Coronagraphy is the only mode for which the contrast plots and corresponding reports are available in the Plots pane and Reports pane, respectively. 

The contrast is calculated in the following manner:

  1. Circular aperture extraction, with a radius equal to the Aperture radius, is performed along the Contrast azimuth, centered on the Contrast separation. The Sky annulus is also measured radially outward from the position of the Contrast separation.

  2. The scene is then offset so that the central star is no longer occulted. The same Aperture radius and Sky annulus values are used for circular aperture extraction, this time centered on the star.

  3. The contrast is computed as the flux extracted along the Contrast azimuth at the Contrast separation divided by the flux extracted from the unocculted star.

Latest updates
    Updated for ETC 3.0.

    Updated for ETC 2.0. Added information on the contrast calculation.

  • Updated for ETC 1.7.

    Updated for ETC 1.5.

  • Updated for ETC 1.3.

    To reflect changes in the ETC 1.2.2 patch release (PSF subtraction options).
Originally published